The Gallium nitride (GaN)-based blue light emitting diode (LED) developed by Nichia Chemical Co., Ltd. in 1993 uses a GaN p-n thin film junction to provide blue and green LED devices, and in 1997, a short wavelength (404 nm) blue LED having a life span of about 10,000 hours at room temperature has been developed using a nitride semiconductor.
Recently, an n-type oxide semiconductor, zinc oxide (ZnO), has attracted attention as an another efficient light emitting (λ=380 nm) material at room temperature due to its interesting features: (1) a direct transition band structure, (2) a low power threshold for optical pumping at room temperature and (3) a large exciton binding energy (60 meV).
However, the development of a p-n heterojunction structure of zinc oxide has been hampered due to the difficulty of fabricating p-type ZnO.